A general description of the sherardizing process can be found in monographs or handbooks, for example, "Corrosion and Protection of Metals" by Bakhvalov and Turkovskaya, Pergamon Press, 1965, or "Zincification", handbook edited by Proskurkin, Moscow, Metallurgy, 1988.
The typical sherardizing process is described in these and others handbooks, as including the following general steps:
preparing of articles surface by chemical or shot blasting treatment; PA1 placing of cleaned articles into a drum, filled with zinc powder and inert filler zinc oxide, sand, aluminum oxide and others). The amount of inert filler and zinc powder can be varied from 1:10 to 1:1; PA1 sealing of the drum and heating up to the temperature within the range 380-450 degrees of Celsius and isothermal heating for 1-4 hours;
In order to accelerate the process the drum can be rotated with a small velocity;
After completing the heating step the coated articles are discharged from the drum and transferred to finishing operation, usually passivation.
The coating received by this process consists of several phases, defined by different zinc-iron ratio depending on heating temperature and time and composition of zinc-containing mixture. The coating may have thickness from 5 to 200 and more microns. The coating color is usually gray, and after passivation it becomes dark-gray.
The problem with which usually is associated with sherardizing is melting and coalescence of zinc powder particles due to the fact that Zn melting point is close to the process temperature. Usually this problem is solved by introducing into mixture of an inert additive, providing for physical barrier between the Zinc particles or creating this barrier artificially on the surface of Zn particles.
In SU 1534091 there is disclosed steam-oxidation treatment of Zn powder resulting in creating on Zn particles oxides and hydroxides preventing melting of adjacent particles together. In SU 560001 there is disclosed sherardizing, which is carried out in within the powder mixture, consisting of (wt. %): Cr 10-20, Zn 10-20, silicon oxide 10-15, chromic oxide 10-20, NH.sub.4 Cl 1-3, remainder Al.sub.2 O.sub.3.
In CN 1084582 there is described sherardizing mixture consisting of Zinc, guarzite and small additions of Pb.
The above mentioned approach based on introduction of inert additives into sherardizing mixture overcomes the above mentioned problem and is widely used in industry.
It should be pointed out however that sherardizing within powder mixtures with inert additivies does not improve corrosion resistance in comparison with coatings deposited by alternative technologies and does not allow to control the coating color.
For example corrosion resistance of coatings obtained by sherardizing are worse in conditions of Salt-Spray (the standard environment for determination corrosion stability of coatings) that zinc coatings deposited electrolytically. Despite some improvement of corrosion resistance can be achieved after passivation, it does not always satisfy customers requirements and besides the passivation is associated with deterioration of coating visual appearance because its color becomes dark-gray.
In FR 2451405 there is described that in order to impart brilliance to an article after sherardizing process it is treated by phosphatization and then is polished It should be realized however that in order to provide such a coating with reasonable corrosion resistance it should be passivated. Seeing that passivation results in a colorless pasivating film it can be readily understood that after passivation the brilliant appearance will be lost. It should be also mentioned that phosphatizing with subsequent polishing doesn't provide for an opportunity to control the coating color.